Adhesive bonding is widely used in the aerospace industry and in other fields for joining fibre-reinforced composite components to one another and to metal components. A wide variety of aerospace structural parts such as rotor blades, wings, struts, fan case liners and the like incorporate bonded joints between components. By eliminating or reducing the use of mechanical fasteners, adhesive bonding provides a number of known advantages.
To achieve adhesive bonds providing satisfactory performance and reliability under demanding conditions of use requires not only appropriate selection of adhesive type, surface preparation and the like, but also careful attention to the exact fit of the surfaces being joined. This is particularly important when bonding complementary surfaces of complex three-dimensional shape. It is important that in the eventual joint the gap between the component surfaces at the bonding interface, corresponding to the thickness of a layer of adhesive between them (bondline thickness) is not excessive, or a region of insufficient bond strength may result. Conversely, any actual contact between the components not only loses adhesion at the contacting regions and inhibits adhesive flow but may also hold the components apart at neighbouring regions which would otherwise fit properly. Achieving bondline thickness within a desired range all over the bonding interface is particularly important where exceptional operational stresses are to be expected, or where the components have different degrees of thermal expansion. Another issue particularly relevant for fibre-reinforced composite materials is slight non-smoothness or non-flatness of the composite surface after cure. Especially when one such component is to be bonded to another also having a range of dimensional tolerance, non-uniformities can coincide to take the bondline thickness outside the desired or permissible range.
For these reasons among others, especially with high-value high-performance components, it is conventional to go through a series of fit check procedures before adhesive is actually applied to both surfaces. In a known procedure a first layer of solid (film) adhesive is applied to one component over the bonding interface, covered by a release film, the shaped component surfaces are offered up together to a predetermined relative datum position and then separated. The test film indicates, by a transparency/colour change, regions where the adhesive layer has contacted both surfaces. Further protected adhesive layers are applied at the non-contacting regions and the process repeated, gradually building up a pattern or profile of adhesive layer thickness corresponding to the bondline thickness profile over the interface. Because of the crude sensitivity of the film test (contact or non-contact, without knowledge of the actual remaining clearance) it is impossible to gauge precisely the correct amount and area of additional adhesive, inevitably leading to inexact filling of the bondline; perhaps only 80% by volume. Corresponding iterative non-adhered fit checking of the components may also be needed before any adhesive is applied, to identify surface regions with excessive or inadequate spacing. These regions are reshaped and the procedure repeated until spacing all over the bonding interface is predicted to be within permissible ranges and the adhesive can be applied.
This process is therefore extremely laborious and time-consuming. It would be desirable to provide a more efficient and convenient procedure, and a procedure enabling improved adhesive fill accuracy at the bondline.